The light-emitting diode (LED) is known by its high energy efficiency and is introduced to various fields of use. The available lighting product with a light-emitting diode in market provides not only white light but also variable color lights of almost the whole range of the visible wavelength (400 nm˜750 nm) and ultraviolet. But a phosphor is usually used to convert the light emitted from an LED to a light with a desired wavelength.
White light can be generated in several ways such as by mixing lights from blue, green, and red chips, exciting phosphor by UV light, or exciting phosphor by blue light. The most common commercial way of generating white light is adopting yttrium aluminum garnet (YAG) phosphor pumped by blue light of an LED to generate a complementary color to be mixed with the blue light to generate white light. For example, a 460 nm blue chip emits blue light to excite YAG:Ce phosphor to generate a 570 nm yellow light, and the blue light is mixed with the yellow light to generate white light.
A phosphor is usually doped with an activator, and the activator is usually selected from rare earth elements such as Ce (Cerium) and Eu (Europium). However, in addition to phosphor, the rare earth elements are widely used in many other fields such as Laser, Hybrid vehicles, Superconductors, and positron emission tomography (PET) scan, and the demand for the rare earth elements gets strong, which restrains the availability of the rare earth elements in the near future.